The present invention relates, in general, to an apparatus for adding gas to a liquid polyurethane component, such as a filler-laden polyurethane component, of a polyurethane injection molding assembly.
A conventional gas charging unit of a type is shown in FIG. 1a and includes a mixing vessel 1 and an agitator 2 which is provided at one end with blades 3 and operatively connected to a motor M for rotation in the mixing vessel 1. A feed conduit 4 is connected to the mixing vessel 1 for supply of a component K, such as e.g. polyol or isocyanate with fillers. A further feed conduit 5 connects into the feed conduit 4 for introducing gas G into the liquid component. The gas laden component K+G is discharged through an outlet line 6 at an upper location of the mixing vessel 1.
FIG. 1b shows another conventional mixing vessel 11 which accommodates an agitator 12 driven by a motor M. The agitator 12 includes a hollow shaft 13 and blades 15 which have gas channels 14 in fluid communication with the interior of the hollow shaft 13. A feed conduit 16 is connected to the mixing vessel 16 for introduction of a component K, and an outlet line 17 which branches off the mixing vessel 13 for discharge of the component K laden with gas G. Gas G is drawn from a gas chamber 18 above the liquid level of the component K via the hollow shaft 13 of the rotating agitator 12 and introduced into the component K via the gas channels 14 in the blades 15.
Gas charging units of these types suffer many shortcomings. For one, the mixing energy of the agitators 2, 12 is insufficient to maximize a gas introduction. Moreover, in view of their high speed rotation, the agitators 2, 12 crush the fillers (reinforcement fibers), and an effective sealing is difficult to implement. In addition, the dynamic agitators 2, 12 also heat up the polyurethane component K (polyol or isocyanate) to an unwanted degree. Further, the bubble size of admixed gas is not small enough and not sufficiently homogenous, and flow patterns may be established that cannot be influenced by the agitators 2, 12 and thus represent areas of insignificant mixing action.
As in the start-up of conventional polyurethane injection molding assemblies with static mixer and fixed mixing elements, introduced polyurethane component has not reached the required operating temperature, operation of the polyurethane injection molding assembly becomes impossible in this cold state with such mixers because the intrinsically viscous polyurethane component (isocyanate or polyol) is highly viscous and thus, the assembly would exceed the maximally admissible operating pressure.
It would therefore be desirable and advantageous to provide an improved gas charging unit for filler-laden polyurethane components, to obviate prior art shortcomings and to realize a gas introduction at maximum physically possible gas fraction, while still being gentle to filler constituents.